1. Field of the Invention
The invention relates generally to marine inboard-outboard drive systems, and more particularly, to a vertical trim system for adjusting the vertical height of a marine outdrive.
2. Description of Related Art
Marine inboard-outboard drive systems are well known in the art. A typical inboard-outboard system includes an engine mounted inside a boat that is coupled to an outdrive unit through an opening in the boat transom. A transom plate is coupled to the boat's transom and a seal is provided to seal the opening. The transom plate further supports the outdrive system. The outdrive unit is coupled to the transom plate via a gimbal ring that pivots about a vertical pivot axis for steering purposes. The gimbal ring also allows the outdrive unit to pivot about a horizontal pivot axis for kick-back movement of the outdrive unit.
A driveshaft extends through the opening in the transom, with one end of the driveshaft coupled to the engine inside the boat, and the other end coupled to the outdrive unit so as to turn a propeller shaft. In known inboard-outboard systems, the driveshaft is coupled to the outdrive via a universal joint to allow the outdrive unit to pivot via the gimbal ring for steering or kick-back.
Propeller location is very significant in any marine drive system. A key principle for performance is that the propellers be located at their optimum depth in the water at all times, providing optimum efficiency, speed and control of the boat. A problem arises when mounting an outdrive of an inboard-outboard system to the transom of a boat. Outdrives are fixed at the engine crankshaft height. Once the outdrive is secured to the transom, it is not vertically adjustable. Therefore, the propeller depth also may not be adjusted once the outdrive unit is secured to the transom. When installing an inboard-outboard drive system, an optimum propeller depth relative to the boat is determined. Then, the system is installed such that the propeller is located at this optimum depth. This propeller depth, however, is only "optimum" for a given set of conditions, since the optimum propeller depth changes as conditions change.
Several factors affect the propeller depth necessary for optimum boat performance. For example, the total weight and center of gravity of the boat changes as fuel is consumed or the number of passengers changes. Sea conditions also affect the desired propeller location--a propeller depth that is appropriate for calm seas likely will not provide optimum performance in choppy seas. Thus, the "optimum" propeller depth calculated prior to installing the outdrive is a compromise, at best. This problem is magnified in dual-drive system boats that have a V-hull design. Since the engines are placed side-by side, they are located higher above the water line in order to fit into the V-hull. The propellers, in turn, are also mounted higher, affecting the trim capability of the boat.
Unfortunately, no satisfactory solution to the above described problem exists in the prior art. In one attempted solution, spacer blocks are placed between the outdrive upper case and lower foot, effectively extending the depth of the outdrive unit, thereby extending the propeller further into the water. Spacers, however, cannot be used to raise the propeller height relative to the boat. Moreover, adding or removing spacers is a complicated and time-consuming undertaking, and if conditions change, the boat's driver cannot change the propeller depth while the boat is underway.
Another attempted solution uses a "set-back," or spacer box. The outdrive is mounted on the spacer box, which is located between the boat's transom and the outdrive gimbal ring, rather than mounting the outdrive directly to the transom. The position of the spacer box may be manually adjusted to a small degree, which in turn, allows the propeller depth to be varied slightly. However, as with the spacer blocks, the propeller depth may be varied only a small amount, and the process is time consuming and expensive. Furthermore, the box assembly cannot be adjusted for changing conditions while the boat is underway.
The present invention addresses these, and other, shortcomings of prior art marine outdrive systems.